PURPOSE Because a family history of cancer constitutes an important risk factor, estimation and communication of cancer risk can facilitate efforts toward early detection and prevention. This review provides a resource for health professionals called to draw upon a multidisciplinary literature to provide quantitative risk estimates to families with cancer. DESIGN Descriptive population-derived, epidemiologic, genetic, genetic epidemiologic, and molecular studies are critically reviewed in the context of cancer genetic counseling. RESULTS Data are presented that document the increased lifetime relative risk to relatives of individuals with cancers of the breast, ovary, colon, prostate, or other sites. In general, risk ratios are poorly suited for clinical counseling. Age-specific absolute risks are presented for first-degree relatives of individuals affected by cancers of the breast, ovary, and colon. The derivation of Mendelian and Bayesian risk estimates in the setting of well-defined cancer family syndromes, and the growing role of DNA testing in more accurately assessing these risks, are discussed. Such carrier testing requires careful psychologic and ethical considerations. CONCLUSION Multidisciplinary cancer genetic counseling is an emerging resource available to physicians who care for families with common adult malignancies.